In Pokémon Gold & Silver, Genders were created to allow for breeding of the Pokémon. Genders have since become a staple of the Pokémon World. There were no differences between Genders in the games, except for the Nidoran line...until now.
Diamond & Pearl have brought the feature of Gender differences to the table. However, this is not utilised for every Pokémon, and not all are visible from the front. Only one fifth of the Pokémon have differences between gender. In the game, if you have seen the differences, you can check in part of the Pokédex as seen in the picture here. However, below is a list of all of the Pokémon with differences and a note as to what the difference is